The use of sodium ricinoleate, the sodium salt of mixed fatty acids from castor oil, in the treatment of periodontal disease and as an effective anti-plaque and anti-calculus agent has long been recognized, such as, for example, as disclosed in Demers, D. G. et al., Journal of Periodontology, Vol. 38, pp 294-301, July-August 1967; Jones, H. E., British Journal of Dental Science, Vol. 70, pp 171-179, 1927 and Hartzell, T. B., Journal American Dental Association, XII, pp 1452-67, December 1925. In addition, in U.S. Pat. No. 1,633,336 a castor oil soap is disclosed as being useful in a dentifrice to render mouth bacteria harmless. The use of sodium ricinoleate in an oral preparation for use as a mouthwash or dental product is disclosed in U.S. Pat. No. 1,936,456 issued to the Wm. S. Merrell Company. Moreover, the Wm. S. Merrell Company marketed such as oral preparation in a toothpaste, powder and mouthwash form under the trademark DETOXOL. The active ingredient of said products was sodium ricinoleate, namely, sodium 12-hydroxy-9-octadecenoate, known as Soricin, the major component of castor oil soap.
The marketing of such products has, however, not been entirely successful due at least in part to the instability and insolubility of sodium ricinoleate in the compositions which results in undesirable haze formation and precipitation. The solubility of sodium ricinoleate and its related fatty acid salts is limited by the pH of the aqueous system they are dissolved in. In water the solubility of sodium ricinoleate decreases with decrease in concentration because the pH is a direct function of fatty acid concentration. The un-ionized species at pH 8 or below in water, are essentially insoluble and therefore show instability. An aqueous solution of sodium ricinoleate at pH 7.9 forms a heavy haze precipitate and is milky white and completely opaque white at pH 7.0 and below.
Soluble salts of ricinoleate and its related salts can be used but they require a very high pH, pH 9-10, and/or solvent concentration to keep the hydrolyzed form in solution. However, both high pH and solvent concentration are unsuitable or undesirable for physiological application. Furthermore, sodium ricinoleate and the related materials are adversely affected by many things which are normally part of a pharmaceutical formulation such as heavy metal ions, cations, ionic salt effect, oxygen and oxidizing agents.
Moreover, in determining the effectiveness of a substance as a plaque inhibitor the contact angle becomes an important determining factor. The contact angle is a measure of the hydrophobicity of a treated surface determined by measuring the angle a sessile drop of water makes with the treated surface. The greater the angle the more hydrophobic the surface is. Sodium ricinoleate has a limited long term solubility in water at concentrations pharmaceutically acceptable at its upper range and clinically effective at the lower range. As a result, within the concentration range where sodium ricinoleate is effective, as demonstrated by contact angle on tooth or hydroxyapatite surface, a physically unacceptable haze or floc is formed in aqueous solution. Other solvents or additives can increase the solubility of sodium ricinoleate but such solutions tend to produce only minimal adsorbed layers of sodium ricinoleate on the tooth surface with low contact angles of reduced durability. For example, in solutions of sodium ricinoleate in alcohol, the contact angle decreases from 135.degree. in water to less than 50.degree. in alcohol. Likewise, although surfactants can increase the solubility of sodium ricinoleate in water, there is, however, a resultant severe reduction in the contact angle of the composition.
There is, therefore, a need for formulation of a special sodium ricinoleate composition that provides a concentration of sodium ricinoleate above the acceptable level in pure water without decrease in its anti-plaque and calculus activity, as exhibited by substantially no loss of or reduced durability of its contact angle or absorbed layer on the tooth surface and with sodium ricinoleate solubility and liquid clarity acceptable for a mouthwash type product.